With the advent of computers came the need for devices which could produce the results of computer generated work product in a printed form. Early devices used for this purpose were simple modifications of the then current electric typewriter technology. But these devices could not produce picture graphics, nor could they produce multicolored images, nor could they print as rapidly as was desired.
Numerous advances have been made in the field. Notable among these has been the development of the impact dot matrix printer. While that type of printer is still widely used, it is neither as fast nor as durable as is required in many applications. Nor can it easily produce high definition color printouts. The development of the thermal ink-jet printer has solved many of these problems. U.S. Pat. No. 4,728,963 issued to S. O. Rasmussen et al., and assigned to the same assignee as is this application, teaches an example of this type of printer technology.
Thermal ink-jet printers operate by employing a plurality of resistor elements to expel droplets of ink through an associated plurality of nozzles. In particular, each resistor element, which is typically a pad of resistive material about 50 .mu.m by 50 .mu.m in size, is located in a chamber filled with ink supplied from an ink reservoir. A nozzle plate, comprising a plurality of nozzles, or openings, with each nozzle associated with a resistor element, defines a part of the chamber. Upon the energizing of a particular resistor element, a droplet of ink is expelled by droplet vaporization through the nozzle toward the print medium, whether paper, fabric, or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements.
The pen containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column on the pen multiplied times the distance between nozzle centers. After each such completed movement or swath, the medium is moved forward the width of the swath, and the pen begins the next swath. By proper selection and timing of the signals to the nozzles, the desired print is obtained on the medium.
In order to obtain multicolored printing, the column of nozzles in the pen can be allocated to the distribution of different colored inks. For instance, a pen with a column of nozzles 48 nozzles in length may be constructed such that the first twelve nozzles can be supplied with cyan ink, the next twelve nozzles can be supplied with magenta ink, the next twelve with yellow ink, and the last twelve with black ink. Using this arrangement, each complete movement or swath of the pen across the medium could print four color bands, each band being twelve nozzle spacings or index positions wide. The medium would then be advanced twelve index positions so that the next swath would have the magenta ink nozzles moving over the same medium positions as were the cyan ink nozzles on the previous swath. By continuing to advance the medium by twelve index positions before each swath of the pen, each of the print positions on the medium could, if directed by the microprocessor, be printed by each of the ink colors. Using this arrangement, any given individual position on the print medium is addressed four times on four consecutive swaths. But the print medium will have advanced twelve index positions between each swath. Therefore, the information from the computer concerning this print position has to be temporarily stored and used on the four consecutive swaths, each of which is separated by twelve index positions. This is referred to as a data index of twelve lines. Using this arrangement, it is possible to produce reasonably high quality multicolored printed images of both alphanumeric characters and graphics at a reasonably high rate of speed.
But thermal ink-jet printer technology is itself not without problems, and considerable need has existed for a means of solving some of these problems. The most obvious problem associated with thermal ink-jet printers has been the tendency of the print produced to be of a less than desirable definition or quality. Highest character definition could be achieved if ink were deposited on the media only where intended if the ink would stay where it is deposited without migrating. Unfortunately, because of phenomena such as that of the wet ink being drawn into the surrounding dry media by capillary action, the edges of the printed characters tend to become less defined. Also, when inks of differing colors are printed adjacent to each other, the different colored inks tend to bleed into each other. Further, the wet ink on print media that have a low absorption rate (i.e., transparency film) tends to clump together in small puddles due to surface tension, thus creating a phenomenon called ink coalescence. Another problem encountered in ink-jet printing is paper cockle. The ink used in thermal ink-jet printing is of a liquid base. When the liquid ink is deposited on wood-based papers, it absorbs into the cellulose fibers and causes the fibers to swell. As the cellulose fibers swell, they generate localized expansion, which, in turn, causes the paper to warp uncontrollably in these regions. This phenomenon is called paper cockle. This can cause a degradation of print quality due to uncontrolled pen-to-paper spacing, and can also cause the printed output to have a low quality appearance due to the wrinkled paper.
Hardware solutions to these problems have been attempted. Heating elements have been used to dry the ink rapidly after it is printed. But this has helped only to reduce smearing that occurs after printing. Prior art heating elements have not been effective to reduce the problems of ink migration that occur during printing and in the first few fractions of a second after printing.
Other types of printer technology have been developed to produce high definition print at high speed, but these are much more expensive to construct and to operate, and thus they are priced out of the range of most applications in which thermal ink-jet printers may be utilized.
To the inventors' knowledge, no prior art solution to the problem of lack of definition in the product of thermal ink-jet printers has been, either singly or in combination with other attempted solutions, successful in bringing the overall print definition of these printers within optimal limits.